Although I am not personally interested in backporting a fix for this issue, anyone that experiences this issue in python 3.5 can execute the following monkey patch to solve the issue:

def _fix_issue_36041_3_5():
    from email._header_value_parser import QuotedString, ValueTerminal, quote_string
    import email._header_value_parser

    class BareQuotedString(QuotedString):

        token_type = 'bare-quoted-string'

        def __str__(self):
            return quote_string(''.join(str(x) for x in self))

        @property
        def value(self):
            return ''.join(str(x) for x in self)

        @property
        def parts(self):
            parts = list(self)
            escaped_parts = []
            for part in parts:
                if isinstance(part, ValueTerminal):
                    escaped = quote_string(str(part))[1:-1]
                    escaped_parts.append(ValueTerminal(escaped, 'ptext'))
                else:
                    escaped_parts.append(part)
            # Add quotes to the first and last parts.
            escaped_parts[0] = ValueTerminal('"' + str(escaped_parts[0]), 'ptext')
            escaped_parts[-1] = ValueTerminal(str(escaped_parts[-1] + '"'), 'ptext')
            return escaped_parts

    email._header_value_parser.BareQuotedString = BareQuotedString